In the past, the following types of thermalconductive sheets with low rigidity have been proposed as a member of a heat dissipation and cooling structure for heat generating electronic parts and the like:
(1) A thermalconductive sheet that is not deformed or damaged by the compression of a heat generating electronic member (referred to below as "chip") and a heat sink, because the compression load on the sheet is held low when the sheet is compressed to establish a tight contact (referred to below as "contact compression") between the chip and the heat sink.
(2) A thermalconductive sheet that is tightly contacted when being contact-compressed between a chip and a heat sink having an uneven surface on the contact side, because the thermalconductive sheet is elastic, thus ensuring a sufficiently large thermalconductive surface.
(3) A thermalconductive sheet that (a) has sufficiently tight contact with all of a plurality of chips mounted on a board, and (b) that is not deformed or damaged by the compression of the chips and a heat sink when being contact-compressed between the plurality of chips and the heat sink, because the thermalconductive sheet is elastic.
A silicone gel sheet containing a thermalconductive filler, and Japanese Tokkai No. Hei 2-166755, No. Hei 2-196453, and No. Hei 6-155517 are known as examples for such thermalconductive sheets.
However, a silicone gel sheet containing a thermalconductive filler, and a thermalconductive silicone gel sheet having grooves on one or both sides (Tokkai Hei 2-166755), are extremely soft and lack strength, and therefore pose severe problems regarding the handling of the sheets during the mounting process. Furthermore, because both sides of the sheet have roughly the same degree of stickiness, the sheet may stick to both the chip and the heat sink when releasing the contact compression between the chip and the heat sink for maintenance after the mounting, resulting in extreme deformation or even rupture, and thus making reuse of the sheet impossible.
A compound thermalconductive sheet, wherein a silicone gel containing a thermalconductive filler is layered and hardened on a silicone rubber sheet containing a thermalconductive filler (Tokkai Hei 2-196453), and a thermalconductive compound sheet, wherein a low rigidity silicone rubber containing a thermalconductive filler is integrated with a sheet made by coating a meshed reinforcement material with a silicone rubber containing a thermalconductive filler and then hardened (Tokkai Hei 2-196453), are both soft in a direction perpendicular to the sheet, rigid in the in-plane direction and sticky only on one side and therefore easy to handle during the mounting process and can be reused after release of the contact compression. However, because the compressibility of such sheets is about 2 to 5 times higher than the compressibility of a single thermalconductive silicone gel sheet, these sheets cannot be used when the mounting electronic parts such as chips, heat sinks, and boards having low rigidity, or when the height difference of a plurality of chips mounted on a board is large.